3 Stacked Bathrooms, Toilet Bubbling When Flushed Above

[NTL1991]

Trying to get to the bottom of a drain issue and hopefully sort things out. First floor toilet bubbles (enough to wet the bottom of the toilet seat lid) when the third floor toilet is flushed. Flushing the 2nd floor toilet only causes some minor disturbance in the 1st floor bowl, similar to that of a windy day. Same deal at the 2nd floor toilet when the 3rd is flushed, no bubbling.

The soil stack is 4" cast iron and vented through the roof. All late 40's cast iron except for the basement portion of the stack which is horribly noisy PVC. House drain is 4" PVC as well.

1st floor toilet 4" closet bend -> 4" reducing wye (lav & tub used to tie in here) -> 4" combination w/ vent line up adjacent to soil stack -> 4" soil stack to house drain

I'm assuming that the second floor also has a similar vented combo, with it's own vent line, and they all combine above the 3rd floor level before the stack is vented through the roof. Does that sound right?

So I'm thinking that the falling water from the 3rd floor is pushing quite a bit of air ahead, and the air is finding the easiest path to be through the water in the 1st floor bowl, right?

That would seem to mean I have a blockage I need to investigate in the soil stack below the 1st floor combo, the house drain or the 1st floor vent line, right?

 

[WorthFlorida]

If this is something new, my first suspicion is the first floor vent has blockage. It could be possible that a bird or other varmint has nested in the vent pipe connection in the attic space.

Vents are to relieve both positive and negative air pressure. When the third floor toilet is flushed, the column of water is rushing down the waste stack. It is pushing air (positive pressure) and the air should be going up the vent of the second floor toilet and then the first floor toilet vent. The air behind the water is a negative pressure so it would be pulling air from the roof vent. At least from your drawing the third floor may not have a separate vent since it maybe a dry vent anyway. If the first floor vent is clogged, the main stack is acting as a wet vent. Should all three be flushed at the same time, then the first floor would be slow to drain or gurgle.

 

[Sponsor]

 

[Reach4]

I am surprised that the stack itself does not do the venting in practice, but anyway, what is going on at B in your photo. Is A the toilet, and B intended as a vent for the toilet, or is B the lavatory drainage, in which case B would be the wet vent from the lavatory.

And what are F and G... is there a side port into the E fitting?
You should be clamping your pex at J and K, but that is not the topic of this thread.

 

[NTL1991]

Thanks guys.

I can only assume that the 2nd and 3rd are vented that way, the walls are not open and there is no attic access.

A is 1st floor toilet
B is where lav and tub drain originally connected. I am almost certain the 2nd and 3rd floor bathrooms are still connected this way.

This is an old pic that I had on hand. That 1/2 PEX at K for a washing machine was leaking at the crimped fittings from a previous repipe. I have replaced it all with ProPEX and secured it along the joists and down the foundation wall to the laundry shutoff with Tapcons.

The 3/4” PEX at J is for a hot water heating system retrofit done by that same plumber. I believe I’ve already resupported it but I will double check.

I believe G to be a vent fitting off the combo, and F a vent line that runs up the plumbing chase, to presumably where it would connect to the stack in the attic. There is no connection between G and E